This invention relates to controlling the braking systems of a towed vehicle by a towing vehicle.
Towing of vehicles such as trucks is known. Typically, a vehicle having mechanical or other problems that prevents its operation is towed by a towing vehicle to a place for repair, etc. A malfunctioning vehicle such as a truck can be towed by having one axle hoisted by a towing truck and the towed vehicle is moved on its other axle.
Trucks are also towed by pulling them with all axles on the ground (road). Trucks are increasingly being equipped with a glad hand structure (such as front glad hands to couple the brake system with the towing vehicle) to facilitate towing of the truck in this manner. In addition to the physical connections made between the towing vehicle and towed vehicle for facilitating towing, the glad hand structure is utilized for controlling the brakes on the towed vehicle from the towing vehicle.
A truck has two types of brakes—service brakes and parking brakes. The service brakes can be separated into primary brakes and secondary brakes. The primary brakes are associated with the drive axle (i.e. the rear wheels). The secondary brakes are associated with the steer axle (i.e. the front wheels). Parking brakes may also be referred to as emergency brakes or as spring brakes. Parking brakes are also associated with the drive axle (i.e. the rear wheels).
In order to permit vehicle motion, pressurized air is used to compress the emergency brake springs (i.e. to release the emergency brake). In order to stop the vehicle using the emergency brake (i.e. to apply the emergency brake), the springs are released by releasing the pressurized air from the spring brake chambers. The air system is used to engage or disengage the brakes on a truck. Pressurized air (or, high pressure air) in the brake system is supplied by a compressor powered by the engine.
A general braking system 100 associated with trucks is illustrated in FIG. 1A. Braking system 100 includes a wet (or supply) tank 105 which receives air that has been pressurized by a compressor. Air from the wet tank 105 is supplied to the primary air tank 110 and the secondary air tank 115. In general, air from the primary tank 110 is used to operate and control the rear brakes (i.e. the drive axle) 150 while air from the secondary tank 115 is used to operate and control the front brakes (i.e. the steer axle) 170. For illustrative purposes, only one rear axle 150 is shown; a truck may have more than one rear axle. The rear axle 150 includes both (rear or primary) service brake chambers 15 and spring brake chambers 25. Air is supplied to each of these chambers via separate lines as illustrated.
The primary tank 110 and the secondary tank 115 supply air to a treadle valve that is actuated by the brake pedal. The treadle valve may include or consist of two pneumatically separate valves such as a top or primary portion TVR 140 and a bottom or secondary portion TVF 160. The portion of the valve closest to the brake pedal, TVR 140, receives air from the primary tank 110 and controls the rear axle brakes 150. The remaining portion of the valve, TVF 160, receives air from the secondary tank 115 and controls the brakes on the front axle 170.
When the brake pedal is depressed, TVR 140 is triggered and a signal is communicated to rear axle relay valve, RV 145, which applies air to the rear brakes 150 to slow or stop the vehicle. The air is supplied from the primary tank 110. Simultaneously, the secondary portion of the treadle valve, TVF 160, is triggered and air is delivered via the quick release valve, QRV 165, to activate the front brakes 170.
Air from the primary tank 110 is supplied via a T fitting 120 (a T fitting is herein after referred to as a T) to TVR 140 and to double check valve DCV 125. Air from the secondary tank 115 is supplied to TVF 160 and double check valve DCV 125. Air from DCV 125 is supplied via T 130 to parking control valve PCV 135 and to spring break relay valve SBRV 190. Air from PCV 135 is also supplied to spring brake control valve SBCV 180. As is known, a double check valve permits air from one of multiple sources to flow based on which source provides the higher pressure—the higher pressure air is permitted to flow.
The parking or emergency brake is controlled by PCV 135. In order to facilitate vehicle motion, the parking brake is released by applying pressurized air from PCV 135 (via SBCV 180 and SBRV 190) to spring brake chambers on rear brakes 150. This pressure releases the rear brakes. The vehicle can be stopped using the parking brake by shutting off pressurized air from PCV 135 to the spring brake chambers. This results in the parking brakes being applied.
Brakes on trucks typically fail due to loss of pressure in the primary circuit (associated with the primary tank). In such a situation, the parking or spring brakes have traditionally been controlled by the park control relay valve (such as PCV 135). More recently, federal regulations specified the use of a spring brake inversion valve to facilitate the control of parking or spring brakes via the treadle rather than having to use the instrument panel-mounted park control valve.
A braking system utilizing a spring inversion valve (which may also be referred to as an inversion valve or a spring brake modulating relay valve) is illustrated in FIG. 1B. If a pressure loss occurs in the primary tank, the spring brake modulating relay valve, SBMRV 185, allows the spring brakes to be applied by the treadle valve 140, 160 rather than having to use the park brake control valve, PCV 135.
When the air brake system is functioning properly, SBMRV 185 monitors delivery pressure from the treadle valve TVR 140 and TVF 160 (via Ts 240 and 260 for example). When the parking control valve, PCV 135, is pushed in to release the spring (or parking) brakes, the modulating relay valve SBMRV 185 opens a path from the secondary tank 115 to the spring brake chambers to keep the spring brakes released to permit vehicle motion.
Air from the primary tank 110 is supplied to the treadle TVR 140 and to PCV 135 via T 230. Air is also supplied to the relay valve for the rear brakes, RV 145, by the primary tank 110 and by TVR 140. Air from the secondary tank 115 is supplied to the treadle TVF 160 and to PCV 135 via T 250. Air from the secondary tank 115 is also supplied to the modulating relay valve SBMRV 185 via a check valve 195. Air from TVF 160 is also supplied to the front brakes 170 via T 240 and quick release valve, QRV 165.
A loss of primary pressure at the spring brake modulating relay valve, SBMRV 185, results in an unbalanced condition inside the valve since secondary control pressure from treadle valve TVF 160 is present without control pressure from the relay valve RV 145. A service brake application (via the treadle) allows pressure from the secondary tank to enter SBMRV 185 in proportion to the amount of force applied to the treadle valve. The modulating relay valve SBMRV 185 then allows a proportionate amount of spring brake hold-off pressure to exhaust from the spring brake chambers.
Towing via glad hands is not compatible for trucks utilizing the spring brake modulating relay valve such as SBMRV 185. Previous front towing glad hand systems utilized only the primary circuit (i.e. the primary tank). In order for the spring brake modulating relay valve to function properly, air from the secondary circuit/tank is also (i.e. in addition to air from the primary tank) required. A need exists, therefore, to provide a braking system for towing a vehicle having glad hands and a spring brake modulating relay valve.